Density
. Density, amount of a substance contained within a specific area. In physics, density is the ratio of the mass of a substance to its volume, and it can be calculated by dividing the mass by the volume. Density is often expressed in units such as grams per cubic centimeter or pounds per cubic foot. The density of a substance can vary under different conditions. Substances expand and contract as their temperature changes, and as a result their density also changes. Precise measurements of density therefore include the temperature at which they were taken. For example, 1 cu cm of water at 4°C (39°F) weighs 1 g, so water’s density at that temperature is 1 g/cm3. Hot air balloons are able to fly because the density of air changes as its temperature changes. The balloon rises because the heated air inside its bag is less dense than the cooler air outside. Pressure also affects the density of gases. If a given amount of gas is contained within a smaller volume, its density increases. Air at